Field of the Invention
The present invention relates to a multilayer ceramic composite containing at least one composite material forming a supporting zone that has oxidation-sensitive reinforcing fibers, and at least one ceramic surface layer.
Carbon-containing composite materials, are used for example for brake applications, and are known in particular from the aviation industry and motor racing sector. They offer the advantage of good tribological properties right up to very high loads and temperatures.
Materials formed of carbon fiber-reinforced carbon materials (CFC or C/C) are used widely in this connection. Such materials contain carbon fibers in the form of mats, woven fabrics or other types of two-dimensional fiber structures that are stacked on top of one another and to form three-dimensional bodies and are then post-compacted with carbon. The post-compaction may be carried out by repeated impregnation with so-called carbon precursor materials (substances that decompose under pyrolysis conditions to form carbon) such as pitches or resins, and their subsequent pyrolysis to form carbon, or by deposition of so-called pyrocarbon from the gaseous phase (CVI, chemical vapor infiltration).
Composite materials for tribological applications and in particular brake discs may be subdivided according to their structure into two zones having different requirement profiles and in most cases also different material properties and composition.
The outerlying zone exposed to wear and co-operating tribologically with a second body (e.g. the brake lining) is the friction surface, which is characterized by special friction and wear properties. The underlying material is the supporting zone (core body), whose task is essentially only to absorb the frictional forces and transmit them to the securement devices, as well as absorb and dissipate the frictional or braking energy.
Composite materials with a ceramics matrix have recently been developed on an increasing scale. In this connection, materials of particular interest are those that are built up from carbon fibers and a matrix of silicon carbide-containing (SiC) or Si/SiC-containing (additional silicon phases) matrices. These materials include in particular the so-called C/SiC materials, which are composed of carbon fibers or carbon-containing fibers and a matrix of carbon (C), Si and SiC. Such composite materials are known, inter alia, from Published, Non-Prosecuted German Patent Application DE 198 56 721, German Patent DE 197 11 829 C1 and Published, Non-Prosecuted German Patent Application DE 197 10 105 A1.
For the material propertiesxe2x80x94in particular the strength and rigidityxe2x80x94it is very important and normal practice to protect the reinforcing fibers by coating them with carbon or carbon-containing compounds.
A common feature of all the specified materials is that they contain carbon in fiber form or in the matrix, and that the materials are heated under use conditions to temperatures at which a noticeable oxidation of the carbon takes place during prolonged use. The oxidation is assisted by the fact that the listed materials, in general in the original statexe2x80x94i.e. without post-treatment and/or additional protective measures for fibers or matrix phases containing elementary carbon have a not insignificant open porosity. On account of oxidation and the loss of carbon caused thereby the structure is weakened and the strength is reduced. In the case of brake discs, the weakening may be very deleterious especially as regards the functioning of the supporting zone since a material failure in the region of the device for securing the brakes or discs can lead to a total failure of the assembly.
The effect of this oxidative damage can be monitored for example by the weight loss of the composite materials during use.
For this reason composite materials for tribological applications and in particular high-performance brake discs with core bodies or supporting zones of carbon-containing material must include effective anti-oxidation mechanisms for the supporting zone. Since the physical properties, in particular the coefficient of thermal expansion, and the chemical properties of different CFC or different C/SiC or Si/SiC materials (silicon carbide-containing materials infiltrated with silicon) may differ greatly, the effectiveness of every such anti-oxidation system also varies greatly.
Various solutions have been proposed to provide antioxidation protection.
The application of anti-oxidation surface layers is one of the most commonly employed methods. Specifically in the case of CFC and C/SiC composites it is of particular importance for the anti-oxidation protection that these materials are permeated by a fine crack structure that can expand and close under alternating thermal loads. In addition, new cracks are also generated under mechanical stress.
Accordingly it is particularly those anti-oxidation protective layers that are self-healing that are of great interest. The mechanism of self-healing is based on the property that the protective layers melt at the application temperature and newly-formed cracks can reseal.
Such systems are described for example in European Patent EP 0 619 801 B1 and Published, European Patent Application EP 0 375 537 A1. The disadvantage with these systems is the fact that the protective layer formed as a glass layer has to act as a carrier of the self-healing properties in order to protect the supporting zone specifically on the surface layer, i.e. the outermost surface. On account of the softening and/or melting of this layer at the application temperature, the frictional behavior is seriously impaired.
Another possibility for crack sealing under the application conditions is to add high melting point elements, binary or multinary compounds, for example boron compounds, that oxidize at elevated temperatures under the admission of air at least partially to form oxides such as B2O3 and/or low melting point glasses such as borate glasses. A double-ply cover layer system on a CFC body is described in U.S. Pat. No. 5,536,574. The CFC body is coated with a mixture of Si, SiC and Al2O3 and then borated with a boron-containing mixture. A second boron-containing layer is then applied. In this case too the outermost layer softens at the application temperature and accordingly the functioning of the tribologically active surface cannot be guaranteed.
Furthermore, the B2O3 glasses formed from the boron-containing compounds have turned out to be extremely harmful for the frictional properties of the cover layer.
As a further variant, the use of a boron-containing Si/SiC matrix has been proposed in U.S. Pat. No. 5,962,103. In this way cracks lying deep in the composite material can also be healed by the melting of the boron compounds formed during oxidation.
A disadvantage in use however is that the matrix properties, in particular the stiffness and strength, deteriorate markedly due to the formation of low melting point B2O3 glasses in the event of oxidation and under high temperature conditions.
Those reinforcing fibers and/or matrix materials that are oxidatively degraded at elevated temperature (i.e. at temperatures that may arise during use and that are above approximately 400xc2x0 C.) are termed oxidation sensitive and may therefore lead to a weakening of the composite material. In particular carbon as a constituent of the matrix or in fiber form is oxidation sensitive, in which connection the oxidation sensitivity of the latter can be reduced in a known manner (in particular according to the teaching of Published, Non-Prosecuted German Patent Application DE 197 10 105, the relevant contents of which are incorporated by reference in the disclosure) by suitably coating the fibers.
It is accordingly an object of the invention to provide a multilayer ceramic composite and a process for forming the composite that overcome the above-mentioned disadvantages of the prior art compositions and methods of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a multilayer ceramic composite. The ceramic composite has at least one composite material forming a supporting zone and contains oxidation-sensitive reinforcing fibers. The composite material includes a matrix containing a mass proportion of at least 25% SiC, further phases being phases having Si or phases having Si alloys, and/or carbon being either carbon in elemental form or a carbon compound. At least one ceramic surface layer having reinforcing fibers is provided. The ceramic surface layer includes a matrix containing a mass proportion of at least 25% SiC, further phases being either phases having Si or phases having Si alloys, and/or carbon being either carbon in elemental form or a carbon compound. At least one protective layer having a matrix composed substantially of at least one component of the matrix of the supporting zone or the ceramic surface layer, is provided. The protective layer is disposed between the supporting zone and the ceramic surface layer. The protective layer contains additives whose oxides are low melting point glasses having a melting point of at most 1250xc2x0 C.
The object of the present invention is accordingly to protect the supporting zone or core body of a composite ceramic reinforced with oxidation-sensitive fibers that may also contain oxidation-sensitive fractions in the matrix, in particular of carbon fiber-reinforced SiC, C/SiC or Sixe2x80x94SiC, against oxidative damage, in particular due to entry of air, by a self-healing and crack-sealing materials without having to alter the composition and properties of the surface layer of a composite ceramic joined thereto, which is provided in particular for frictional wear and which moreover is exposed to an entry of air.
The present invention achieves the object by the provision of the multilayer composite ceramic body that includes between the supporting zone and the surface layer a protective layer that is generically related to the composite material and contains the additives for the formation of self-healing layers. As additives there may be used in particular in this connection various boron-containing compounds and their oxidation products, or alkali metal aluminosilicates.
The term xe2x80x98generically relatedxe2x80x99 is understood in this context to mean that the matrix of the relevant layer is built up substantially from at least one of the matrix phases of the supporting zone or of the surface layer. The matrix composition of the relevant layer may, in this connection, still contain elements from the elements contained in the additives.
Thus for example in the preferred case of a supporting zone built up from fiber-reinforced C/SiC the matrix of the relevant generically related layer may be built up substantially from Si and/or SiC. In the case where the composite body is built up from a supporting zone of a CFC material and a surface layer of Sixe2x80x94SiC, the relevant layer would then be generically related if its matrix were built up predominantly from C, or Si and/or SiC.
The present invention accordingly provides a multilayer ceramic composite containing at least one supporting zone or supporting layer that includes oxidation sensitive reinforcing fibers as well as a matrix, with it also being possible for the matrix to contain oxidation-sensitive fractions, and at least one surface layer, characterized in that the surface layer contains at least one additional generically related protective layer situated between the supporting zone and surface layer and that contains additives that form self-healing layers.
Preferably the layers of the multilayer composite have the external shape of circular discs or flat cylinders. Such a rotationally symmetrical shape is necessary for the preferred use as friction discs (brake discs or coupling discs).
In accordance with an added feature of the invention, the compositions of the composite material of the ceramic surface layer and of the supporting zone are different.
In accordance with another feature of the invention, the reinforcing fibers are formed of carbon fibers, graphite fibers and/or carbon-containing fibers.
In accordance with an additional feature of the invention, the additives of the protective layer are high melting point elements, binary compounds or multinary compounds that have a melting point of at least 1450xc2x0 C.
In accordance with a further feature of the invention, the additives are boron, non-oxidic boron compounds, boron carbide, calcium boride, boron silicide, aluminium boride, titanium boride, zirconium boride, alkali metals, alkaline earth metals, aluminum compounds, or oxides of the aluminum compounds.
In accordance with a further added feature of the invention, the protection layer has reinforcing fibers, and a volume proportion of the reinforcing fibers in the protective layer is below 80%.
In accordance with a further additional feature of the invention, the protective layer has a thickness of 0.02 to 5 mm.
In accordance with another further feature of the invention, the support zone has a given thickness, and the ceramic surface layer has a thickness that is less than 50% of the given thickness of the supporting zone.
In accordance with another added feature of the invention, the multilayer ceramic composite has a shape of a cylindrical disc with a given thickness. The ceramic surface layer has a thickness less than 20% of the given thickness of the cylindrical disc, and the thickness of the ceramic surface layer is at least 0.2 mm.
In accordance with another additional feature of the invention, a further protective layer is disposed on the supporting zone, and a further surface layer is disposed on the further protective layer.
With the foregoing and other objects in view there is provided, in accordance with the invention, a process for producing multilayer ceramic composites. The process includes providing a carbon-containing fiber-reinforced precursor for forming a supporting zone, providing at least one carbon-containing precursor for forming a surface layer, and bonding the carbon-containing fiber-reinforced precursor forming the supporting zone to the carbon-containing precursor forming the surface layer surface wide using an adhesive composition resulting in a bonded body. The adhesive composition during pyrolysis, produces a porous, carbon-containing layer and the adhesive composition contains glass-forming additives such as admixtures of high melting point elements, binary compounds, or multinary compounds in each case having melting points of at least 1450xc2x0 C., and due to oxidation the glass-forming additives produce low melting point glasses with a melting point of at most 1250xc2x0 C. The bonded body is pyrolyzed for forming a carbon-containing multilayer composite infiltrated with silicon or silicon alloys, and at least part of the carbon reacting with the silicon to form silicon carbide.
In accordance with an added mode of the invention, there is the step of setting a mean particle size of the glass-forming additives to be less than 120 xcexcm.
In accordance with another mode of the invention, there is the step of bringing a silicon-containing melt into contact with the bonded body exclusively over a surface layer.
In accordance with a further mode of the invention, after an infiltration and reaction with the silicon, the bonded body is heated air or an oxygen-containing gas mixture for a sufficient time at an appropriate temperature, whereupon the glass-forming additive is at least partially converted into a low melting point oxidic glass. The heating is carried out at an appropriate temperature until a mass fraction of oxide phases in the adhesive composition transformed into a protective layer is 1 to 80%.
With the foregoing and other objects in view there is provided, in accordance with the invention, a process for producing multilayer ceramic composites. The process includes filling a compression mold with at least three molding compositions of different compositions that constitute precursors of a supporting zone, a protective layer and a surface layer. The molding compositions for the supporting zone and the surface layer contain carbon fibers and the molding composition leading to a formation of the protective layer contains a glass-forming additive being admixtures of high melting point elements, binary compounds, or multinary compounds each with melting points of at least 1450xc2x0 C. and, due to oxidation, the glass-forming additive produces low melting point glasses with a melting point of at most 1250xc2x0 C. The three molding compositions are compressed in the compression mold resulting in a compressed body. The compressed body is pyrolyzed which results in a carbon-containing multilayer composite infiltrated with silicon or silicon alloys, and at least part of the carbon reacts with the silicon to form silicon carbide.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a multilayer ceramic composite, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.